1) Field of the Invention
The present invention relates to a process for producing refined pyromellitic acid to be used as a raw material of coating agent, etc., and a process for producing refined pyromellitic anhydride to be used as a raw material of polyimide resins with high heat resistance, cross-linking agents for foam polyester and particular plasticizers.
2) Prior Art
As processes for producing pyromellitic acid, a process for producing pyromellitic acid by liquid phase oxidation of durene and a process for producing pyromellitic acid by liquid phase oxidation of 2,4,5-trimethyl benzaldehyde have been known. As a process for producing pyromellitic anhydride by dehydration refining of crude pyromellitic acid, a process comprising dehydrating crude pyromellitic acid in the presence of aliphatic acid anhydride such as acetic anhydride has been known. However, the above-mentioned process possesses a defect of a high cost since acetic anhydride is used.
Further, as a process for producing pyromellitic anhydride, a process comprising performing gas phase of durene or 2,4,5-trimethyl benzaldehyde has been known.
Japanese Patent Kokai (Laid-open) No.62-59280 discloses a process for producing pyromellitic anhydride comprising heating pyromellitic acid at a specific temperature.
Pyromellitic acid and pyromellitic anhydride produced by the above-mentioned processes contain a small amount of by-products such as trimellitic acid (TMA) and methyl trimellitic acid (MTMA) and have been often colored. Although Japanese Patent Kokai (Laid-open) No.62-59280 discloses to dehydrate pyromellitic acid recystallized with water, pyromellitic anhydride thus obtained is a little colored. Therefore, further a high quality of pyromellitic anhydride and pyromellitic acid has been required.
An object of the present invention is to provide a process for producing refined pyromellitic acid and refined pyromellitic anhydride which little contain by-products and are not colored.
As a result of extensive studies to solve the above-mentioned prior art problems, the inventors have found that a high quality of refined pyromellitic acid with a large particle diameter not to be colored can be produced with an industrial advantage by dissolving colored crude pyromellitic acid or crude pyromellitic anhydride containing by-products with heating in water and then cooling an aqueous solution thus obtained to perform crystallization as pyromellitic acid, then separating a crystal thus obtained to obtain pyromellitic acid little containing impurities, then heat dehydrating pyromellitic acid thus obtained, then vaporizing pyromellitic anhydride thus obtained, cooling the vapor and recovering a refined crystal of pyromellitic anhydride and furthermore pyromellitic acid not to be colored, little containing impurities can be obtained by dissolving crude pyromellitic acid or crude pyromellitic anhydride with heating in water, performing contact treatment of an aqueous solution thus obtained with activated carbon, then separating the aqueous solution from activated carbon, then cooling the aqueous solution to perform crystallization as pyromellitic acid and separating the crystal, and a refined pyromellitic anhydride can be readily produced by heat dehydrating pyromellitic acid thus obtained at a temperature of 170 to 260xc2x0 C., and have accomplished the present invention.
That is, the present invention provides a process for refined pyromellitic anhydride which comprises:
dissolving crude pyromellitic acid or crude pyromellitic anhydride in water,
then, cooling an aqueous solution thus obtained to perform crystallization as pyromellitic acid,
then, separating a crystal thus obtained from water,
then, anhydrating the crystal of pyromellitic acid thus separated with heating to produce pyromellitic anhydride,
then, vaporizing pyromellitic anhydride thus produced,
cooling vapor of pyromellitic anhydride thus obtained, and
thereby, recovering a refined crystal of pyromellitic anhydride.
Further, the present invention provides a process for producing refined pyromellitic acid which comprises:
dissolving crude pyromellitic acid or crude pyromellitic anhydride in water,
then, performing a contact treatment of an aqueous solution thus obtained with activated carbon,
then separating the aqueous solution from activated carbon,
then, cooling the aqueous solution to perform crystallization as pyromellitic acid,
then, separating a crystal thus obtained from water, and
thereby, recovering refined pyromellitic acid.
The present invention provides a process for producing refined pyromellitic anhydride which comprises:
dissolving crude pyromellitic acid or crude pyromellitic anhydride in water,
then, performing a contact treatment of an aqueous solution thus obtained with activated carbon,
then separating the aqueous solution from activated carbon,
then, cooling the aqueous solution to perform crystallization as pyromellitic acid,
then, separating a crystal thus obtained from water to recover refined pyromellitic acid, and
dehydrating pyromellitic acid thus recovered with heating at a temperature of 170 to 260xc2x0 C. to obtain refined pyromellitic anhydride, or
further vaporizing pyromellitic anhydride thus produced,
cooling vapor of pyromellitic anhydride thus obtained, and
thereby, recovering a refined crystal of pyromellitic anhydride.